The power to move a motor vehicle comes from burning fuel in an engine. Emissions from vehicles are the by-products of this combustion process. In addition, VOC escape through fuel evaporation. As vehicle exhaust systems have improved, evaporative emissions have become a larger component of total-vehicle VOC emissions.
The combustion process results in emissions of VOC, NOx, PM, and CO, which are released from the tailpipe while a vehicle is operating. Exhaust emissions occur during two modes:
VOC also escape into the air through fuel evaporation. Despite evaporative emissions controls, evaporative losses can still account, on hot days, for a majority of the total VOC pollution from current model cars. Evaporative emissions occur in several ways:
Source: U.S. Environmental Protection Agency. Automobile Emissions: An Overview. Fact Sheet OMS-5.
Starting a car cold increases trip emissions compared to starting an engine that is already warm. A typical automobile on the road in 2002 had an average trip length of 4.0 miles, and, with slightly more than 7 trips per day, an average of about 29 vehicle miles traveled per day. On a given weekday, cold starts of a typical vehicle produces 7.7 grams of VOC (25 percent of the typical daily emissions), 88 grams of CO (26 percent of the typical daily emissions), and 5 grams of NOx (19 percent of the typical daily emissions). Running exhaust accounts for another 7.8 grams of VOC, 251 grams of CO, and 20.2 grams of NOx.
VOC are also emitted through fuel evaporation. For example, parking the car all day produces 4.3 grams of VOC.
Source: U.S. Environmental Protection Agency. MOBILE6.2 Model run assumed IDLE Test, National Low Emission Vehicle Standards, summer temperature 64-92 degrees, and United States average vehicle operations. 20 April 2004.
Emissions rates vary based on the speed a vehicle is traveling. EPA's model for highway vehicle emissions - MOBILE 6.2 - shows how speed affects emissions rates. VOC and CO emissions rates typically drop as speed increases. NOx emissions rates turn up at higher speeds. Emissions rates at all speeds have been falling over time as newer, more controlled vehicles enter the fleet.
*Note: summer, freeway assumption used.
**Note: winter, freeway assumption used.
These curves do not represent the full range of effects associated with travel at different speeds. Emissions rates are higher during stop-and-go, congested traffic conditions than free flow conditions operating at the same average speed.
Source: U.S. Environmental Protection Agency. MOBILE 6.2 Model run 24 September 2003.
These comparisons show estimated in-use emissions rates, in grams/mile, for cars and heavy-duty diesel trucks with 2002 control technology versus 1967 vehicles (before significant controls). Car emissions rates have declined by 80 percent to 95 percent depending on pollutant, while heavy-duty diesel truck emissions rates have declined by 82 percent for VOC, 42 percent for NOx, and 63 percent for CO.
Source: U.S. Environmental Protection Agency. MOBILE6.2 Model run, 2 March 2005.
A small percentage of vehicles emit a large percentage of the pollution from on-road vehicles. These "gross emitters" include not only older model cars, but also some new cars with poorly maintained or malfunctioning emissions control equipment. As shown in the diagram, it is estimated that less than 10 percent of the vehicle fleet emits approximately 50 percent of the VOC emissions. The same vehicles, however, are not always gross emitters for all criteria pollutants - different 10 percent may be gross emitters for CO, NOx , and others. Additionally, 10 percent to 27 percent of the vehicles failing inspection never end up passing the state inspection and maintenance tests.
Source: National Academy Press. Evaluating Vehicle Emissions Inspection and Maintenance Programs. (July 2001): 27-29, 33 (Copyright 2001).